Cordless telephone station sets are commercially available which employ a radio transmission link between a portable hand-held unit and a fixed base station unit. Some of these telephone sets operate over a particular radio frequency channel. In cellular mobile radio, the mobile set bids for access to a plurality of radio frequency channels. Access to channels is provided by a centralized controller through a transceiver at a cell site. The problem with radio telephone systems generally is the limited availability of radio frequency spectrum, the undesirability of interference between transceivers operating at similar frequencies, and, hence, the insecurity of any communications.
To overcome these problems, cordless telephone sets have been proposed which employ line-of-sight transmission which may be defined as transmission at frequencies above approximately one gigahertz. In particular, the safety to the user and other considerations have pointed development in the area of cordless telephony towards infrared transmission.
It has been reported that a cordless infrared telephone set has been designed at Siemens Aktiengesellschaft, Munich, West Germany. The set comprises a wall-mounted stationary unit and a hand-held telephone set equipped with a dialing keyboard and a control keyboard of three keys. A user may use the hand-held set anywhere in a room, unconfined by cords. Interference with transmission to wall-mounted stationary units by sunlight and other infrared sources is overcome to a degree by mounting the stationary unit near or on the ceiling of a room and by employing infrared-reflecting window panes which are often installed to save energy in modern buildings.
It has also been reported that a data communications system has been developed at IBM Zurich Research Laboratory which incorporates a plurality of cordless data terminals capable of operation at 125 Kbits/s. It is further reported that a 125 Kbits/s optical data transmission link has been built and a switched in-house data network designed which comprises a host and cluster controllers connected by conventional house cable. The cluster controller is connected by house cable to a number of rooms where a ceiling mounted satellite controls access to a number of data terminals over the optical link. Communications are reasonably secure in accordance with the above discoveries. Telephone rewiring of a premises is required when telephone stations are moved from one room to another.
In U.S. Pat. No. 4,275,385, a personnel locater system is disclosed comprising portable transmitters. Each transmitter is personal to an individual who carries it and transmits a unique code. A master processor is capable of identifying the vicinage of a transmitter as it identifies which ceiling mounted receiver has reported the code. In this manner, local alerters in the vicinage of the received transmission can alert a needed individual. Only one way transmission is disclosed. A portable transmitter, in accordance with the disclosed system, is capable of movement outside the boundaries of a room, unlike the communications devices of the IBM and Siemens disclosures.
It is generally believed advantageous to maintain maximum flexibility in telephone systems. It is desirable that office and telephone station rearrangements occur without expensive rearrangements of conventional telephone house cable. Records as to features, services and locations of particular telephones within a particular system should be centrally maintainable without human intervention. Once a premises is equipped with a plurality of ceiling mounted transceivers, no further intervention should be required when a telephone is moved within the network. A portable telephone station set should be capable of being carried about a premises. In the process of being moved from room to room, the portable set should permit an on-going telephone conversation to continue without interference or concern about the confidentiality of the communication.
Furthermore, it is well known that a one channel to one station ratio is uneconomical and impractical in operation. A means for allocating line-of-sight transmission channels among a number of cordless stations is required for the practical implementation of a cordless key telephone system.